The following application is related to the present application: U.S. patent application Ser. No. 09/549,422 entitled xe2x80x9cMETHOD AND APPARATUS FOR ASYMMETRICALLY INDUCING VOLTAGES IN TRANSFORMER SECONDARY WINDINGS WHILE AVOIDING SUTARATION OF THE TRANSFORMER CORE,xe2x80x9d naming Daniel F. Mulhauser, as the inventor, assigned to the assignee of the present invention, and filed concurrently herewith.
The present invention relates generally to switching circuits and, more particularly, to circuits that may be used for switching transistors at high frequencies and high voltages.
Switching circuits have been designed for applications in which switching devices must stand off and supply high voltages, and in which rapid switching (e.g, in the range of microseconds or faster) is required. One of these applications, connecting a traveling wave tube to its high voltage cathode supply, is described in U.S. Pat. No. 4,754,176 to Jones, et al. As noted in Jones, switching transistors are preferred in these applications, as compared, for example, to mechanical relays, due to the requirements for rapid switching. In addition, it may be desirable to employ a number of switching transistors in series in order to overcome limitations on the amount of voltage that a single device can handle. Connecting the switching transistors in series typically imposes the additional requirements that the driving circuits of the transistors be electrically isolated from each other, and that the switching be synchronous. Jones accomplishes the isolation and synchronous switching of series-connected transistors by employing one transformer for turning the switches on (labeled 200 in FIG. 2, driven by transistor Q1), and another transformer for turning the switches off (unlabeled, driven by transistor Q2).
Another application in which high voltages must be rapidly switched is in the use of external heart defibrillators. These devices supply controlled electrical pulses that are applied to the chests of patients in cardiac arrest. Defibrillators may also be implanted, in which case the electrical pulses are applied directly to the heart and the voltages to be switched naturally are much smaller. Older external defibrillators typically used mechanical relays as the switching devices. Defibrillators that are more modern typically use solid state switching circuits having power transistors to switch the high voltages. These power transistors may be metal-oxide semiconducting, field-effect transistors (MOSFET""s), insulated gate bipolar transistors (IGBT""s), or similar known devices.
In one aspect of the present invention, a switching circuit is disclosed that includes one or more switching devices, one or more detector and driver circuits, and at least one control and driving signal provider (hereafter, simply xe2x80x9csignal providerxe2x80x9d). The signal provider provides one or more control and driving signals at one or more output ports. The signal provider may also have one or more input ports for accepting one or more primary control signals. The control and driving signals are based at least in part on the primary control signals. In some implementations, a first output port of a first signal provider is electrically isolated from a second output port of the first signal provider. Also, a first in put port of a first signal provider may be electrically isolated from a first output port of the first signal provider.
The detector and driver circuits each have an input coupled to an output port of the signal provider. The detector and driver circuits each also have an output coupled to at least one of the switching devices. Each detector and driver circuit detects when a control and driving signal at its input is in an on state and, responsive thereto, drives at least one of the switching devices on by applying to it the control and driving signal. In addition, each detector and driver circuit detects when the control and driving signal is in an off state and, responsive thereto, drives at least one of the switching devices off by applying to it the control and driving signal.
Thus, for each detector and driver circuit of this aspect of the invention, the same signal (the control and driving signal) performs both the function of controlling the turning on and off of the switching devices and the function of driving the switching devices on and off in response to the controlling function. In contrast, conventional circuits typically employ separate signals for controlling and driving, and/or they employ one signal for controlling and driving the switching device on and another signal for controlling and driving the switching device off. For example, in the circuit described in Jones (referred to above), the pulses generated by the three gates U3 associated with Q1 on the primary sides of the two single-turn transformers provide the turning on (through Q1) control function. The driving signal magnetically induced on the secondary side of transformer 200 turns the switching transistors Q3-Q10 on. The pulses generated by the three gates U3 associated with Q2 on the primary sides of the two single-turn transformers provide the turning off (through Q2) control function. The driving signal magnetically induced on the secondary side of, the unlabelled transformer turns the switching transistors Q3-Q10 off. Thus, separate circuits and signals are used to drive the switching transistors on and off.
In some aspects of the present invention, the signal provider may be a transformer. In these aspects, the input ports of the signal provider are primary windings on the primary side of the transformer, and the output ports of the signal provider are secondary windings on the secondary side of the transformer.
In some implementations, the signal provider consists of a single signal provider. The word xe2x80x9csinglexe2x80x9d in this context means only one signal provider, as contrasted with two or more. In particular, in an implementation in which the signal provider is a transformer, a single transformer may be used to turn the switching devices on and off, rather than the two transformers used, for example, by Jones. In this single-transformer implementation of the present invention, advantages are therefore gained in terms of expense, weight, and volume as compared to the two-transformer circuit described in Jones.
Conventional switching circuits are known that employ single transformers. In particular, FIGS. 1 and 2 of U.S. Pat. No. 5,939,927 to Myers show switching circuits having only one transformer. However, in these conventional circuits, the same signal is not used both to control the off state and to drive the switching device off. Rather, as described in Myers, the switching device is turned off when current in the secondary winding of the transformer is reversed, thereby turning on a depletion mode transistor that causes the gate of the switching transistor to discharge and thus cause the switching transistor to turn off. The switching transistor therefore is not driven off (either by the control signal or, another signal), but, rather, is enabled to discharge into an off state. Also, the circuit in Myers requires that an xe2x80x9conxe2x80x9d state be followed by an xe2x80x9coffxe2x80x9d state, and that the period of the xe2x80x9conxe2x80x9d state be within particular time constraints as established by the design values. The range of possible xe2x80x9conxe2x80x9d time is determined by the magnetizing inductance of the transformer, the level at which its core saturates, and the voltage applied to its primary. An xe2x80x9coffxe2x80x9d state must follow an xe2x80x9conxe2x80x9d state in this conventional circuit because when the primary ceases to be driven the field collapses. This collapsing field drives the secondary in the reverse direction, thereby allowing the depletion mode transistor to turn on, thus turning off the switching device.
The conventional circuits described in Myers generally require a transformer having a large magnetizing inductance that acts to slow the rate of increase of primary current. The current may thus be held below a level at which the core saturates even though the primary is excited during the entire xe2x80x9conxe2x80x9d time of the switching device. In contrast, aspects of the present invention drive the switching device on and off without requiring a transformer with large magnetizing inductance. This arrangement is advantageous because a large magnetizing inductance, as in Myers, generally results in a large leakage inductance having series impedance that slows the driving on and off of the switching transistor. Furthermore, if a number of switching transistors are being switched in series, the large magnetizing inductance may result in a skew between the driving on or off of one or more of the switching transistors as compared to one or more of the other switching transistors. That is, the switches may not all switch at substantially the same time. In that case, the switches may not properly share the voltage across the series connection during transitions between xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d states.
Another conventional circuit that employs a single transformer is described in U.S. Pat. No. 5,781,040 to Myers. As shown in FIG. 2, the switching device of this circuit is not driven to the xe2x80x9coffxe2x80x9d state, as is the case with respect to aspects of the present invention. Actively driving the switching device off generally is advantageous because faster switching times can be reliably achieved. In addition, the circuit described in the ""040 patent relies on a change in control-signal frequency to distinguish on and off control, and thus does not lend itself to rapid switching. That is, because time inherently is required to distinguish one frequency from another (i.e., the differences in periods cannot be ascertained until the periods have passed), switching delays are inherent.
In aspects of the present invention, the switching devices may include one or more transistors. In various implementations, these one or more transistors may include one or more field-effect transistors, one or more insulated-gate bipolar transistors, one or more MOS controlled thyristors (MCT""s), or similar transistor devices. These examples are intended to be illustrative rather than limiting, and any other known switching device, or one to be developed in the future, may be used in other implementations of the present invention.
As noted, the signal provider may include a transformer having one or more primary windings on the primary side and one or more secondary windings on the secondary side. In some aspects of the invention, the input of a first detector and driver circuit is coupled to a first of the secondary windings, and a first control and driving signal includes a voltage waveform across the first secondary winding. The first detector and driver circuit may include an on-threshold detector that detects that the first control and driving signal is in the xe2x80x9conxe2x80x9d state when its voltage reaches an on-threshold voltage. The on-threshold detector may include a zener diode or other voltage reference. Furthermore, the first detector and driver circuit may include an off-threshold detector that detects that the first control and driving signal is in the xe2x80x9coffxe2x80x9d state when its voltage reaches an off-threshold voltage. The off-threshold detector may also include a zener diode or other voltage reference. In some aspects of the present invention, the on-threshold and off-threshold voltages may be of opposite polarities, and the zener diodes of the on-threshold and off-threshold detectors may each be coupled in parallel with the first secondary winding and in opposing polarities with respect to each other.
In some aspects of the present invention, a detector and driver circuit includes a driver circuit that drives a switching device on responsive to the on-threshold detector detecting that a control and driving signal is in the xe2x80x9conxe2x80x9d state. The driver circuit also drives the switching device off responsive to the off-threshold detector detecting that the control and driving signal is in the xe2x80x9coffxe2x80x9d state. The driver circuit includes a first driving switch that couples the control and driving signal to the switching device so as to turn it on when the on-threshold detector detects that the first control and driving signal is in the on state. The driver circuit also includes a second driving switch that couples the control and driving signal to the switching device so as to turn it off when the off-threshold detector detects that the first control and driving signal is in the off state. The first and second driving switches are field-effect transistors in some implementations of the present invention, although any of numerous other known switching devices, or ones to be developed in the future, may also be used.
The present invention, in some aspects, includes a first detector and driver circuit that detects when a first control and driving signal including a voltage waveform across a first secondary winding on the secondary side of the transformer is in an on state and, responsive thereto, applies the first control and driving signal to drive a first of the switching devices on. A second detector and driver circuit detects when a second control and driving signal including a voltage waveform across a second secondary winding is in an on state and, responsive thereto, applies the second control and driving signal to drive a second of the switching devices on. The first and second detector and driver circuits may be electrically isolated from each other. The first and second switching devices may be turned on synchronously.
The word xe2x80x9csynchronouslyxe2x80x9d is used broadly in the context of turning the switching devices on or off to refer to a timing relationship. This relationship may be that of turning the first and second switching devices on (or off) at substantially the same time. Although this particular timing relationship often is advantageous, the timing relationship denoted herein by the word xe2x80x9csynchronouslyxe2x80x9d is not limited to this example. For instance, the second switching device may be turned on (or off) at a time after the first switching device is turned on (or off). That is, a delay in the switching may be introduced in accordance with any of a variety of known techniques. Thus, the words xe2x80x9csynchronously,xe2x80x9d xe2x80x9csynchronized,xe2x80x9d and grammatical variants thereof, generally encompass implementations in which a first switching device is turned on when a second switching device is turned off, or vice versa. In these implementations, the switching devices may be referred to as being in opposite phases, as compared to being in the same phase when they both are turned on at substantially the same time and are turned off at substantially the same time.
In some aspects of the present invention, the first and second switching devices may be coupled to each other in series. These switching devices may be insulated-gate bipolar transistors, or field-effect transistors, in which case the emitter or source of the first switching device may be coupled to the collector or drain of the second switching device, the first control and driving signal may be applied to the gate of the first switching device, and the second control and driving signal may be applied to the gate of the second switching device. However, as noted, the invention is not limited to these examples; the first and/or second switching devices may be any of a variety of other known, or yet to be developed, switching devices.
In some aspects of the invention, a controller applies to a first primary driven winding on the primary side of the transformer a first set of voltages, thereby generating (i) a first current in the first primary driven winding, (ii) a first magnetic field having a first quantum of energy, and (iii) a magnetically induced second set of voltages in the first secondary winding. The controller interrupts the first current, thereby causing the first magnetic field to collapse, and, not later than interrupting the first current, clamps the first primary driven winding to a third set of voltages, thereby magnetically inducing a fourth set of voltages in the first secondary winding. At least one of the fourth set of voltages is less than at least one of the second set of voltages, optionally by a predetermined amount. In some implementations, at least one of the fourth set of voltages may be less than each of the second set of voltages. In yet further implementations, each of the fourth set of voltages may be less than each of the second set of voltages. The first control and driving signal includes the second and fourth sets of voltages. The xe2x80x9conxe2x80x9d state of the first control and driving signal may include one or more of the second set of voltages.
The switching circuit in accordance with these aspects of the invention may include a driver circuit that drives the first switching device on when the on-threshold detector detects that the first control and driving signal comprises one or more of the second set of voltages. The first switching device may remain on when the first control and driving signal comprises one or more of the fourth set of voltages. As described below with respect to one illustrative implementation, the lower values of the fourth set of voltages as compared to the second set of voltages are such that the on-threshold detector does not detect the fourth set of voltages as an xe2x80x9conxe2x80x9d state of the first control and driving signal. However, the fourth set of voltages also is not detected by the off-threshold detector as an xe2x80x9coffxe2x80x9d state of the first control and driving signal. Thus, the first switching device may remain on because it has not been driven off. As also described below with respect to one illustrative implementation, energy stored in the control and driving signal provider (which is a transformer in that implementation) when the second set of voltages is induced may be removed during the period when the fourth set of voltages is induced. Thus, excess energy does not build up in the transformer core.
In order to drive the first switching device off, the controller applies to a second primary driven winding a fifth set of voltages having polarities opposite to polarities of the first set of voltages. In some implementations, the first and second primary driven windings may be the same winding. This fifth set of voltages thereby generates (i) a third current in the primary driven winding, (ii) a third magnetic field having a third quantum of energy, and (iii) a magnetically induced sixth set of voltages in the first secondary winding having polarities opposite to polarities of the second set of voltages. In addition, the controller interrupts the third current, thereby causing the third magnetic field to collapse, and, not later than interrupting the third current, clamps the second primary driven winding to a seventh set of voltages, thereby magnetically inducing an eighth set of voltages in the first secondary winding. The magnitude of at least one of the eighth set of voltages is less than a magnitude of at least one of the sixth set of voltages. The term xe2x80x9cmagnitudexe2x80x9d is used in this context to avoid confusion due to the use of negative values as compared to the voltage values of the first through fourth sets of voltages. In particular, the sixth and eighth sets of voltages may have negative values as compared with the second and fourth sets of voltages, which may illustratively be assumed to have positive values. For example, a voltage value in the sixth set may be xe2x88x9218 volts and a voltage value in the eighth set may be xe2x88x926 volts. The, magnitude of the value of xe2x88x926 volts should be understood to be less than the magnitude of xe2x88x9218 volts, as used herein, even though xe2x88x9218 is a smaller number than xe2x88x926 in the sense that it is more negative. The sixth and eighth sets of voltages are included in the first control and driving signal.
In these aspects of the invention, the controller clamps the second primary driven winding to the seventh set of voltages such that a magnitude of at least one of the eighth set of voltages is less than a magnitude of at least one of the sixth set of voltages by at least a predetermined amount. The driver circuit may drive the first switching device off when the off-threshold detector detects that the first control and driving signal comprises one or more of the sixth set of voltages. The first switching device may remain off when the first control and driving signal comprises one or more of the eighth set of voltages. In particular, the eighth set of voltages does not cause the first switching device to be turned on. The advantage of providing that the first control and driving signal may include the lower-magnitude eighth set of voltages, even though the eight set of voltages does not drive the first switching device off, is similar to that noted above with respect to the inclusion in the first control and driving signal of the fourth set of voltages. That is, energy stored in the control and driving signal provider (i.e., the transformer in one illustrated implementation) may be released during the time when the first control and driving signal includes the eight set of voltages without driving the switching device on.
Thus, in the foregoing aspects of the invention, the xe2x80x9conxe2x80x9d state of the first control and driving signal includes one or more of the second set of voltages, and the xe2x80x9coffxe2x80x9d state of the first control and driving signal includes one or more of the sixth set of voltages. The on-threshold detector detects that the first control and driving signal is in the xe2x80x9conxe2x80x9d state when one or more of the second set of voltages reaches the on-threshold voltage. The off-threshold detector detects that the first control and driving signal is in the xe2x80x9coffxe2x80x9d state when one or more of the sixth set of voltages reaches the off-threshold voltage.
The first set of voltages may include a voltage pulse having a substantially constant amplitude. The fifth set of voltages may include a voltage pulse having a substantially constant amplitude and having opposite polarity to the voltage pulse of the first set of voltages.
The primary windings of the transformer, in some aspects of the invention, include a primary clamp winding. The controller applies the first set of voltages to the first primary driven winding from a voltage supply having an output and a return, thereby generating the first current in a first current path including from the output to the return. The controller provides, not later than interrupting the first current, a second current path for a second current from the return to the output through at least the primary clamp winding. The second current generates a second magnetic field having substantially the first quantum of energy. The controller maintains the second current path for a period of time such that the first quantum of energy is returned to the power supply. The primary clamp winding may include the second primary driven winding. In some implementations, the primary clamp winding has a first number of turns, the first primary driven winding has a second number of turns, and the secondary winding has a third number of turns. A first ratio between the first number and second number, and a second ratio between the first number and the third number, are determined so that a first voltage magnetically coupled to the secondary winding by the first primary driven winding when the first magnetic field is generated is greater than a second voltage magnetically coupled to the secondary winding by the primary clamp winding when the second magnetic field is generated. The first voltage may be greater than the second voltage by at least a predetermined amount.
The circuit in accordance with these aspects of the invention avoids saturation of the transformer core by providing the second current path and allowing the energy in the transformer core to return to the power supply. Moreover, because of the choice of ratios of windings as stated, the voltages magnetically induced on the secondary side of the transformer during the return of energy to the power supply are not large enough to trigger the off-threshold detectors. Thus, the switching devices may remain in the xe2x80x9conxe2x80x9d state during the period when the energy is returning to the power supply. These aspects of the invention therefore provide the advantage, as compared for example with the circuits described in the ""927 patent to Myers (noted above), of cascading xe2x80x9conxe2x80x9d states and thus enabling the switching devices to stay on for extended periods. Conversely, because of the rapid switching times attainable in accordance with aspects of the present invention, the switching devices may be switched on for very short periods in rapid succession. Advantageously, both lengthy xe2x80x9conxe2x80x9d periods and rapid on and off switching are possible without altering component design values.
In additional aspects of the present invention, a method is described for switching one or more switching devices. The method includes the steps of: (a) detecting when a first control and driving signal is in an on state; (b) responsive to step (a), driving at least one of the switching devices on by applying to it the first control and driving signal; (c) detecting when the first control and driving signal is in an off state; and, (d) responsive to step (c), driving at least one of the switching devices off by applying to it the first control and driving signal. Step (a) may include detecting that the first control and driving signal is in the on state when its voltage reaches an on-threshold voltage. Step (c) may include detecting that the first control and driving signal is in the off state when its voltage reaches an off-threshold voltage.
In some implementations, this method may also includes the steps of (e) detecting when a second control and driving signal is in an on state; and, (f) responsive to step (e), applying the second control and driving signal to drive a second of the switching devices on. In these implementations, the first and second switching devices may be turned on synchronously. Also, in some implementations, the method includes the steps of (e) detecting when a second control and driving signal is in an off state; and, (f) responsive to step (e), applying the second control and driving signal to drive a second of the switching devices off. In these implementations, the first switching device may be turned on, and the second switching device is turned off, synchronously.
In accordance with the method noted above with respect to steps (a) through (d), some implementations may also include the step of (e) prior to step (a), applying a first set of voltages to a primary driven winding of a transformer, thereby generating (i) a first current in the primary driven winding, (ii) a first magnetic field having a first quantum of energy, and (iii) a magnetically induced second set of voltages in a first secondary winding on the secondary side of the transformer. These implementations also include the steps of (f) prior to step (c), interrupting the first current, thereby causing the first magnetic field to collapse; and (g) not later than interrupting the first current and prior to step (c), clamping the primary driven winding to a third set of voltages, thereby magnetically inducing a fourth set of voltages in the first secondary winding. At least one of the fourth set of voltages is less than at least one of the second set of voltages. The first control and driving signal includes the second and fourth sets of voltages. Step (g) may include clamping the primary driven winding to the third set of voltages such that at least one of the fourth set of voltages is less than at least one of the second set of voltages by at least a predetermined amount. The xe2x80x9conxe2x80x9d state of the first control and driving signal may include one or more of the second set of voltages.
The above aspects and implementations of the present invention are not necessarily inclusive or exclusive of each other and may be combined in any manner that is non-conflicting and otherwise possible, whether they be presented in association with a same, or a different, aspect or implementation of the invention. The description of one aspect is not intended to be limiting with respect to other aspects. In addition, any one or more function, step, operation, or technique described elsewhere in this specification may, in alternative aspects, be combined with any one or more function, step, operation, or technique described in the summary. Thus, the above aspects are illustrative rather than limiting.